1. Field of the Invention
The present invention relates to a magneto-optical recording medium for use in a magneto-optical system for performing a reproducing operation by using a Kerr effect.
2. Description of the Related Art
In recent years, enlargement of the quantity of information has caused a requirement to arise in that also a magneto-optical recording disc for use as a data storage device has a larger capacity.
A most direct method of raising the density of the magneto-optical recording disc is to reduce the size of a spot formed by reproducing light, that is, to shorten the wavelength of the laser beam. Since shortening of the wavelength of the laser beam, however, encounters difficulties, satisfactory shortening of the wavelength has not been achieved as compared with raising of the density of the magneto-optical recording disc.
Therefore, the density of the magneto-optical recording medium has been realized by reducing track pitches and by raising the linear density rather than by reducing the diameter of the laser beam by shortening the wavelength of the laser beam. Moreover, deterioration in the quality of the signal caused from the foregoing means is modified by employing signal processing techniques including PRML (Partial Response Maximum Likelihood).
On the other hand , attempts have energetically been made in recent years to obtain a resolution higher than that determined by the wavelength of the laser beam.
One of the foregoing techniques has been reported, that is, a magnetically induced super resolution (which is hereinafter called an "MSR") technique which uses magnetic switched connection. The foregoing technique includes a method (Front Aperture Detection which is hereinafter called a "FAD") with which magnetization directions in hot portions of the beam spot are made coincide with one another. Thus, detection of the signal is performed in only cold portions. Another method (Rear Aperture Detection which is hereinafter called "RAD") is included in the foregoing techniques, the method having steps of making magnetization directions in cold portions coincide with one another (masked) and detecting the signal in only hot portions. Since both of the methods encounter reduction in the reproducible area, an effect can be obtained which is the same as that obtainable from reduction in the reproducing beam.
In either case, the intensity of the signal is weakened, causing the tolerance of the system against change in the reproduced signal to become unsatisfactory.
When a reproduced signal in the form as shown in FIG. 1 has a local change (signal imbalance) as shown in FIG. 2 the error rate is raised in the portion in which the change has occurred. As the amplitude of the reproduced signal is reduced or as the local change is made to be great raising of the error rate becomes conspicuous.
When transparent substrates of usual magneto-optical recording discs are manufactured in quantity, the manufacturing cost is reduced. Therefore, disc substrates obtained by injection-molding a polymer material, such as polycarbonate., are widely used.
In general, the plastic substrate has photoelasticity. Therefore, if local dispersion of internal stresses in the substrate causes change in birefringence to occur. The local change in the birefringence results in signal imbalance of an MO signal.
Therefore, a magneto-optical system of a type having a raised recording density and thus suffering from weaker intensity of the signal must prevent the signal imbalance caused from the birefringence.
When the track pitches are reduced, there arises a problem of crosstalk of signals recorded on the adjacent tracks. Since the crosstalk is considerably affected by the warp of the magneto-optical recording disc, the warp of the magneto-optical disc must be prevented as the track pitch is reduced. In a case of a magneto-optical disc having a single-plate structure, the disc warps occurs when the environmental temperature and/or humidity has been changed. The foregoing fact is caused from difference in the expansion or the contraction between the two sides of the magneto-optical disc.
As described above, the magneto-optical disc having the single-plate structure must prevent the warp of the disc even if the environmental temperature and humidity are changed.